SOFC system producing reduced atmospheric carbon dioxide using a molten carbonated carbon dioxide pump

ABSTRACT

A solid oxide fuel cell power generation system&#39;s entire output is made up of three streams: water, sequestered carbon dioxide provided into a storage tank, and carbon dioxide depleted air. Thus, the system generates electricity from a hydrocarbon fuel, while outputting substantially no pollutants into the atmosphere and cleaning the air by removing carbon dioxide from the air exhaust stream. Thus, the system outputs cleaner air than it takes in without releasing pollutants into the atmosphere, while generating electricity from a readily available hydrocarbon fuel, such as natural gas.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of fuel cells andmore particularly to fuel cell systems integrated with carbon dioxideremoval components.

Fuel cells are electrochemical devices which can convert energy storedin fuels to electrical energy with high efficiencies. High temperaturefuel cells include solid oxide and molten carbonate fuel cells. Thesefuel cells may operate using hydrogen and/or hydrocarbon fuels. Thereare classes of fuel cells, such as the solid oxide regenerative fuelcells, that also allow reversed operation, such that oxidized fuel canbe reduced back to unoxidized fuel using electrical energy as an input.

An article by M. P. Kang and J. Winnick, entitled “Concentration ofcarbon dioxide by a high-temperature electrochemical membrane cell”,Journal of Applied Electrochemistry, Vol. 15, No. 3 (1985) 431-439,which is incorporated herein by reference in its entirety, describes amolten carbonate fuel cell that is converted into a molten carbonatecarbon dioxide concentrator for carbon dioxide removal. The authorsreport carbon dioxide removal efficiencies of 97% for carbon dioxideinlet concentrations of 0.25%.

SUMMARY

The embodiments of the invention provide a fuel cell system which scrubscarbon dioxide from the atmosphere. The fuel exhaust stream of a fuelcell stack is sent to a carbon dioxide pump, which uses the fuel exhauststream to remove carbon dioxide from the atmospheric reaction airstream, which is then sent to an air inlet of the fuel cell stack. Thecarbon dioxide pump may be a molten carbonate carbon dioxide pump.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a fuel cell system of an embodiment ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one embodiment of the invention, the entire output of a solid oxidefuel cell power generation system consists of three streams: water,sequestered carbon dioxide provided into a storage tank, and carbondioxide depleted air. Thus, the system generates electricity from ahydrocarbon fuel, while outputting substantially no pollutants into theatmosphere and cleaning the atmosphere by removing carbon dioxide fromthe air exhaust stream. Thus, the system outputs cleaner air than ittakes in without releasing pollutants into the atmosphere, whilegenerating electricity from a readily available hydrocarbon fuel, suchas natural gas. In contrast, many prior art fuel cell systems whichoperate on hydrocarbon fuels output various pollutants into theatmosphere. FIG. 1 illustrates how the molten carbonate carbon dioxidepump is used together with a fuel cell system, such as a solid oxidefuel cell system, to scrub carbon dioxide from the air, while generatingelectricity at the same time.

The fuel cell system 100 contains a fuel cell stack 101, such as a solidoxide fuel cell stack (illustrated schematically to show one solid oxidefuel cell of the stack containing a ceramic electrolyte, such as yttriaor scandia stabilized zirconia, an anode electrode, such as anickel-stabilized zirconia cermet, and a cathode electrode, such aslanthanum strontium manganite).

The system also contains a cascaded electrochemical hydrogen pumpseparation unit 1 which electrochemically separates hydrogen from thefuel exhaust stream. The unit 1 may comprise any suitable protonexchange membrane device comprising a polymer electrolyte. The hydrogendiffuses through the polymer electrolyte under an application of apotential difference between anode and cathode electrodes located oneither side of the electrolyte. The anode, cathode, and polymerelectrolyte together comprise a membrane cell. In a cascaded pump,several sets of cells are arranged in process fluid flow series so thatthe exhaust from one set of cells is used as an input for the next setof cells. In each set of at least two cells, at least two cells arearranged in parallel, such that the input stream is divided among thecells in the set. In other words, any one cell in one set is in processfluid flow series configuration with any one other cell in a differentset, but all cells in each set are preferably in process fluid flowparallel configuration with respect to each other. The unit 1 maycontain two or more sets of cells, such as three to five sets of cells.Each set of cells may contain one or more cells, such as one to twentycells. Preferably, but not necessarily, each set contains more cellsthan the set(s) located downstream from it. For example, in a case of aunit 1 having three sets of cells arranged in series, the unit 1separates hydrogen from the exhaust stream in a three step sequence.First, a quantity (X) of fuel exhaust is provided simultaneously to afirst set of cells having for example four cells, and a first portion(A) of hydrogen is separated. Second, a remaining quantity (X−A) of fuelexhaust is provided to a second set of cells having for example twocells, and a second portion (B) of hydrogen is separated. Third, aremaining quantity (X−A−B) of fuel exhaust is provided to the third setof cells having one cell, and a third portion (C) of hydrogen isseparated. The separated hydrogen (A+B+C) is provided into conduit 7through output 8. The remaining portion of the fuel exhaust consistingessentially of carbon dioxide and water is provided into conduit 9through output 10. The total quantity of separated hydrogen (A+B+C) isat least 95%, such as 95% to about 100% of the hydrogen contained in thequantity (X) of fuel exhaust provided to the electrochemical pump 1. Theterm “hydrogen” as used herein excludes hydrocarbon hydrogen atoms. Forexample, hydrogen includes molecular hydrogen (H₂). Preferably, the unit1 comprises a stack of carbon monoxide tolerant electrochemical cells,such as a stack of high-temperature, low-hydration ion exchange membranecells. This type of cell includes a non-fluorinated ion exchange ionomermembrane, such as, for example, a polybenzimidazole (PBI) membrane,located between anode and cathode electrodes. The membrane is doped withan acid, such as sulfuric or phosphoric acid. An example of such cell isdisclosed in US published application US 2003/0196893, incorporatedherein by reference in its entirety. These cells generally operate in atemperature range of above 100 to about 200 degrees Celsius. Thus, theheat exchangers in the system 100 preferably keep the fuel exhauststream at a temperature of about 120 to about 200 degrees Celsius, suchas about 160 to about 190 degrees Celsius. Other hydrogen separators mayalso be used. However, an additional carbon dioxide scrubber may berequired to be used in conjunction with alternative hydrogen separatorsto remove any remaining carbon dioxide from the hydrogen stream.

The system 100 also contains the first conduit 3 which operativelyconnects a fuel exhaust outlet 103 of the fuel cell stack 101 to a firstinlet 2 of the unit 1. The system also contains a second conduit 7 whichoperatively connects a first outlet 8 of the unit 1 to a fuel inlet 105of the fuel cell stack 101. Preferably, the system 100 lacks acompressor which in operation compresses the fuel cell stack fuelexhaust stream to be provided into the unit 1. The system 100 alsocontains a third conduit 9 which operatively connects a second outlet 10of the unit 1 to an exhaust waste containment unit 21, such as a carbondioxide storage tank for sequestering exhaust waste, such as carbondioxide and/or water. Preferably, the conduit 9 is also connected to adryer 20 that separates the carbon dioxide from the water contained inthe exhaust stream. The dryer 20 can use any suitable means forseparating carbon dioxide from water, such as separation based ondifferences in melting point, boiling point, vapor pressure, density,polarity, or chemical reactivity. Preferably, the separated carbondioxide is substantially free of water and has a relatively low dewpoint. Preferably, the separated carbon dioxide is sequestered in thecontainment unit 21 in order to minimize greenhouse gas pollution by thesystem 100.

The system 100 also contains a carbon dioxide pump 30 which removescarbon dioxide from the air inlet stream provided to the stack 101 andthus removes carbon dioxide from the atmosphere. The carbon dioxide pump30 may include a molten carbonate fuel cell or fuel cell stack that isadapted to remove carbon dioxide from an atmospheric reaction air streamthat is sent to the cathode side of the molten carbonate fuel cell orcells in the stack. The molten carbonate fuel cell may be operated as amolten carbonate carbon dioxide concentrator, as described in thearticle by M. P. Kang and J. Winnick, entitled “Concentration of carbondioxide by a high-temperature electrochemical membrane cell”, Journal ofApplied Electrochemistry, Vol. 15, No. 3 (1985) 431-439, which isincorporated herein by reference in its entirety. The molten carbonatefuel cell includes a cathode, an anode, and an electrolyte. Theelectrolyte of the molten carbonate fuel cell may include a ceramicmatrix that supports a salt mixture, such as a salt mixture of lithiumcarbonate and potassium carbonate, which in operation forms a conductiveliquid. The carbon dioxide pump 30 may also include a carbon dioxidescrubber, such as sodium hydroxide-coated silica, soda lime, or othercarbon dioxide absorbents.

The system 100 also contains a fourth conduit 34 which operativelyconnects the fuel exhaust outlet 103 of the stack 101 to a first inlet31 of the carbon dioxide pump 30. Preferably, the system contains a flowcontrol device 32, such as an orifice or a valve which in operationcontrols the amount of flow of the fuel exhaust stream through conduits34 and 35. The device 32 allows at least a portion of the fuel exhauststream, for example about 3-10%, such as about 5%, to flow into thecarbon dioxide pump 30 via the conduit 34, while allowing the remainderof the exhaust stream, for example about 90-97%, such as about 95%, toflow into conduit 35. For example, when device 32 comprises an orifice,such an orifice is positioned in conduit 34 to narrow the diameter ofthis conduit 34 and to provide the majority of the fuel exhaust streaminto wider conduit 35. The carbon dioxide pump 30 also includes an airinlet 33 through which the atmospheric reaction air stream enters thepump 30. Another conduit 36 operatively connects an air exhaust outlet38 of the carbon dioxide pump 30 to an air inlet 40 of the SOFC stack101. A conduit 42 operatively connects a fuel exhaust outlet 46 of thecarbon dioxide pump 30 to the first inlet 2 of the unit 1. Preferably,conduit 42 provides the fuel exhaust stream from pump 30 into the watergas shift reactor 128. While a molten carbonate fuel cell or stack ispreferred as a carbon dioxide pump, other carbon dioxide pump types maybe used to scrub carbon dioxide from the air inlet stream.

The system 100 further preferably contains a fuel humidifier 119 havinga first inlet operatively connected to a hydrocarbon fuel source, suchas the hydrocarbon fuel inlet conduit 111, a second inlet operativelyconnected to the fuel exhaust outlet 103, a first outlet operativelyconnected to the fuel cell stack fuel inlet 105, and a second outletoperatively connected to the dryer 20. In operation, the fuel humidifier119 humidifies a hydrocarbon fuel inlet stream from conduit 111containing the recycled hydrogen using water vapor contained in a fuelcell stack fuel exhaust stream. The fuel humidifier may comprise apolymeric membrane humidifier, such as a perfluorosulfonic acid membranehumidifier (e.g., NAFION® membrane), an enthalpy wheel or a plurality ofwater adsorbent beds, as described for example in U.S. Pat. No.6,106,964 and in U.S. application Ser. No. 10/368,425, which publishedas U.S. Published Application No. 2003/0162067, all of which areincorporated herein by reference in their entirety. NAFION®, aregistered trademark of DuPont, is a sulfonated tetrafluoroethylenebased fluoropolymer-copolymer. For example, one suitable type ofhumidifier comprises a water vapor and enthalpy transfer NAFION® based,water permeable membrane available from Perma Pure LLC. The humidifierpassively transfers water vapor and enthalpy from the fuel exhauststream into the fuel inlet stream to provide a 2 to 2.5 steam to carbonratio in the fuel inlet stream. The fuel inlet stream temperature may beraised to about 80 to about 90 degrees Celsius in the humidifier.

The system 100 also contains a recuperative heat exchanger 121 whichexchanges heat between a portion of the stack fuel exhaust streamprovided through conduit 35 and the hydrocarbon fuel inlet stream beingprovided from the humidifier 119. The heat exchanger helps to raise thetemperature of the fuel inlet stream and reduces the temperature of thefuel exhaust stream so that it may be further cooled downstream and suchthat it does not damage the humidifier.

If the fuel cells are external fuel reformation type cells, then thesystem 100 contains a fuel reformer 123. The reformer 123 reforms ahydrocarbon fuel containing inlet stream into hydrogen and carbonmonoxide containing fuel stream which is then provided into the stack101. The reformer 123 may be heated radiatively, convectively and/orconductively by the heat generated in the fuel cell stack 101 and/or bythe heat generated in an optional burner/combustor, as described in U.S.patent application Ser. No. 11/002,681, filed Dec. 2, 2004, whichpublished as U.S. Published Application No. 2005/0164051, incorporatedherein by reference in its entirety. Alternatively, the externalreformer 123 may be omitted if the stack 101 contains cells of theinternal reforming type where reformation occurs primarily within thefuel cells of the stack.

Optionally, the system 100 also contains an air preheater heat exchanger125. This heat exchanger 125 heats the air inlet stream being providedto the fuel cell stack 101 using the heat of the fuel cell stack fuelexhaust. Preferably, the exchanger 125 is located upstream of the carbondioxide pump 30. If desired, this heat exchanger 125 may be omitted. Ifthe preheater heat exchanger 125 is omitted, then the air inlet streamis provided directly into the air inlet 33 of the carbon dioxide pump 30by a blower or other air intake device.

The system 100 also preferably contains an air heat exchanger 127. Thisheat exchanger 127 further heats the air inlet stream being provided tothe fuel cell stack 101 using the heat of the fuel cell stack air (i.e.,oxidizer or cathode) exhaust. Preferably, the exchanger 127 is locateddownstream of the carbon dioxide pump 30. The system also optionallycontains a hydrogen cooler heat exchanger 129 which cools the separatedhydrogen stream provided from unit 1, using an air stream, such as anair inlet stream.

The system may also contain an optional water-gas shift reactor 128. Thewater-gas shift reactor 128 may be any suitable device which converts atleast a portion of the water in the fuel exhaust stream into freehydrogen. For example, the reactor 128 may comprise a tube or conduitcontaining a catalyst which converts some or all of the carbon monoxideand water vapor in the fuel exhaust stream into carbon dioxide andhydrogen. Preferably, the reactor 128 lowers the concentration of theremaining methane and carbon monoxide in the fuel exhaust stream totrace levels, such as less than about 1,500 ppm. Thus, the reactor 128increases the amount of hydrogen in the fuel exhaust stream. Thecatalyst may be any suitable catalyst, such as a iron oxide or achromium promoted iron oxide catalyst. The reactor 128 may be locatedbetween the fuel heat exchanger 121 and the air preheater heat exchanger125.

The system 100 operates as follows. A fuel inlet stream is provided intothe fuel cell stack 101 through fuel inlet conduit 111. The fuel maycomprise any suitable fuel, such as a hydrocarbon fuel, including butnot limited to methane, natural gas which contains methane with hydrogenand other gases, propane, methanol, ethanol or other biogas, or amixture of a carbon fuel, such as carbon monoxide, oxygenated carboncontaining gas, such as ethanol, methanol, or other carbon containinggas with a hydrogen containing gas, such as water vapor, H₂ gas or theirmixtures. For example, the mixture may comprise syngas derived from coalor natural gas reformation.

The fuel inlet stream passes through the humidifier 119 where humidityis added to the fuel inlet stream. The humidified fuel inlet stream thenpasses through the fuel heat exchanger 121 where the humidified fuelinlet stream is heated by a portion of the fuel cell stack fuel exhauststream. The heated and humidified fuel inlet stream is then providedinto a reformer 123, which is preferably an external reformer. Forexample, reformer 123 may comprise a reformer described in U.S. patentapplication Ser. No. 11/002,681, filed on Dec. 2, 2004, which publishedas U.S. Published Application No. 2005/0164051, incorporated herein byreference in its entirety. The fuel reformer 123 may be any suitabledevice which is capable of partially or wholly reforming a hydrocarbonfuel to form a carbon containing and free hydrogen containing fuel. Forexample, the fuel reformer 123 may be any suitable device which canreform a hydrocarbon gas into a gas mixture of free hydrogen and acarbon containing gas. For example, the fuel reformer 123 may comprise anickel and rhodium catalyst coated passage where a humidified biogas,such as natural gas, is reformed via a steam-methane reformationreaction to form free hydrogen, carbon monoxide, carbon dioxide, watervapor and optionally a residual amount of unreformed biogas. The freehydrogen and carbon monoxide are then provided into the fuel (i.e.,anode) inlet 105 of the fuel cell stack 101. Thus, with respect to thefuel inlet stream, the humidifier 119 is located upstream of the heatexchanger 121 which is located upstream of the reformer 123 which islocated upstream of the stack 101.

The air or other oxygen containing gas (i.e., oxidizer) inlet stream ispreferably provided into the stack 101 through a heat exchanger 127,where it is heated by the air (i.e., cathode) exhaust stream from thefuel cell stack. Prior to passing through the exchanger 127 and enteringinto the stack 101, the air inlet stream is depleted of carbon dioxideby the carbon dioxide pump 30. For instance, the air inlet stream isdepleted of at least 90%, preferably at least 97%, of carbon dioxidecontained in the ambient air. If desired, the air inlet stream may alsopass through the hydrogen cooler heat exchanger 129 and/or through theair preheat heat exchanger 125 to further increase the temperature ofthe air before providing the air into the stack 101. Preferably, no fuelis combusted with air, and if heat is required during startup, then therequisite heat is provided by the electric heaters which are locatedadjacent to the stack 101 and/or the reformer 123.

Once the fuel and air are provided into the fuel cell stack 101, thestack 101 is operated to generate electricity and a hydrogen containingfuel exhaust stream. An electrical current is produced by transferringoxygen ions from the oxidizer (i.e., cathode) inlet stream through theceramic electrolyte layer of the stack 101 to the fuel (i.e., anode)inlet stream according to the following reactions:Cathode: O₂+4e ⁻→2O²⁻Anode: H₂+O²⁻→H₂O+2e ⁻CO+O²⁻→CO₂+2e ⁻

About 25% of the input fuel exits the fuel exhaust outlet 103 of thestack. The fuel exhaust stream (i.e., the stack anode exhaust stream) isprovided from the stack fuel exhaust outlet 103 to the orifice or valve32 which splits the stack fuel exhaust stream into a first exhauststream portion and a second exhaust stream portion. The first exhauststream portion is provided into the cascaded electrochemical pumpseparation unit 1 through conduit 35, water gas shift reactor 128 andheat exchangers 121 and 125. The second SOFC stack exhaust streamportion is provided into the carbon dioxide pump 30 via conduit 34. Ifthe device 32 comprises a valve, then the valve may be switched toprovide the entire SOFC stack exhaust stream into conduit 35 to by passthe pump 30 in one position and to split the stream between conduits 34and 35 in another position. If the device 32 comprises an orifice, thenit restricts the amount of the stack fuel exhaust stream provided topump 30 through conduit 34. Preferably a minor portion of the fuelexhaust stream, such as 3% to 10%, such as about 5% of the fuel exhauststream is provided into the carbon dioxide pump 30 through conduit 34and the balance is provided to conduit 35.

The SOFC stack fuel exhaust stream is provided into the first inlet 31of the carbon dioxide pump 30 from conduit 34. If a molten carbonatefuel cell or stack is used as the pump 30, then the SOFC stack fuelexhaust stream is provided to the anode of the molten carbonate fuelcell or cells in order to polarize the cells using carbon monoxide andhydrogen contained within the portion of the SOFC fuel exhaust streamprovided to the pump 30. An oxidizer inlet stream, such as atmosphericair, is provided to the cathode of the molten carbonate fuel cell orcells. For example, atmospheric air contains a carbon dioxideconcentration of 0.02% to 0.06%, such as about 0.035%. The air streamcan contain higher concentrations of carbon dioxide, such as about0.25%. An electric current is produced by transferring carbonate ions(CO₃ ²⁻) from the cathode, through the molten carbonate electrolyte, tothe anode according to the following reaction:Cathode: O₂+2CO₂+4e ⁻→2CO₃ ²⁻Anode: H₂+CO₃ ²⁻→H₂O+CO₂+2e ⁻CO+CO₃ ²⁻→2CO₂+2e ⁻

The power produced by the molten carbonate fuel cell is about 200 W andcan be dissipated as heat or can be used to supply power to systemcomponents, such as a control system and/or system display components.The system 100 operating with a 100 kW SOFC stack 101 and integratedwith a 200 W molten carbonate fuel cell pump 30 can remove at least6,500 pounds of carbon dioxide from the atmosphere each year. Theadditional fuel consumed by the system 101 for electrochemicallyremoving the carbon dioxide from the air inlet stream and stack fuelexhaust stream is on the order of 0.3%, as compared to a system withoutsuch carbon dioxide removal capability. Thus, the pump 30 generatespower and removes carbon dioxide from the air inlet stream provided tothe SOFC stack 101.

The remaining fuel exhaust exiting outlet 46 of the carbon dioxide pump30 is provided to the water gas shift reactor 128 via conduit 42. Theair exhaust of the carbon dioxide pump 30 exits the air exhaust outlet38 and is provided to the air inlet 40 of the SOFC stack 101. The airexhaust of the carbon dioxide pump 30 is depleted of carbon dioxide, forinstance contains at least 90% less, such as 97% less carbon dioxidethan the air inlet stream, and is preferably oxygen rich. Optionally,the air exhaust is provided to the air heat exchanger 127 which furtherheats the air inlet stream provided to the fuel cell stack 101.

At least 95% of hydrogen contained in the fuel exhaust stream providedto the unit 1 is separated in the unit 1, for example at least 99%, suchas about 100%, of the hydrogen contained in the fuel exhaust stream isseparated in the unit 1. All hydrogen remaining in the fuel exhauststream except for the hydrogen that is converted to water in the pump 30may be separated in unit 1. The hydrogen separated from the fuel exhauststream in the unit 1 is then provided back into the fuel inlet stream.Preferably, the hydrogen is provided back into the fuel inlet conduit111 upstream of the humidifier 119. The exhaust stream is provided tothe dryer 20 which separates carbon dioxide from water. The separatedcarbon dioxide is provided through conduit 22 for sequestration in tank21. For example, if the fuel cell stack 101 comprises a solid oxideregenerative fuel cell stack, then with the aid of a Sabatier reactor,the sequestered carbon dioxide can be used to generate a hydrocarbonfuel, such as methane, when the stack 101 operates in the electrolysismode, as described in U.S. Pat. No. 7,045,238, incorporated herein byreference in its entirety. The separated water from dryer 20 isavailable for humidification of the fuel inlet stream or otherindustrial uses. For example, conduit 23 may provide the water from thedryer 20 back into the humidifier 119, into a steam generator (notshown) and/or directly into the fuel inlet conduit 111.

The SOFC stack fuel exhaust stream is provided into the unit 1 asfollows. The fuel exhaust stream may contain hydrogen, water vapor,carbon monoxide, carbon dioxide, some unreacted hydrocarbon gas, such asmethane and other reaction by-products and impurities. The portion ofthis exhaust stream is first provided into the heat exchanger 121, whereits temperature is lowered, preferably to less than 200 degrees Celsius,while the temperature of the fuel inlet stream is raised. If thewater-gas shift reactor 128 and the air preheater heat exchanger 125 arepresent, then both portions of the SOFC stack fuel exhaust stream frompump 30 and from heat exchanger 121 are provided through the reactor 128to convert at least a portion of the water vapor and a majority of theresidual carbon monoxide into carbon dioxide and hydrogen. The fuelexhaust stream is then passed through the heat exchanger 125 to furtherlower its temperature while raising the temperature of the air inletstream. The temperature may be lowered to 120 to 180 degrees Celsius forexample.

The fuel exhaust stream is then provided into inlet 2 of the unit 1 viaconduit 3. During the separation step in unit 1, at least a majority ofthe hydrogen, such as at least 95% of the hydrogen in the fuel exhauststream, diffuses through the electrolyte of the cells in the unit 1,while allowing the water vapor, carbon dioxide, and any remaining traceamounts of carbon monoxide and hydrocarbon gas in the fuel exhauststream to be passed through conduit 9 to the humidifier 119. Preferably,the unit 1 separates at least 99% of the hydrogen in the fuel exhauststream, such as about 100%.

In the fuel humidifier 119, a portion of the water vapor in the fuelexhaust stream is transferred to the fuel inlet stream to humidify thefuel inlet stream. The hydrocarbon and hydrogen fuel inlet streammixture is humidified to 80 C to 90 C dew point. The remainder of thefuel exhaust stream is then provided into the dryer 20. The dryer 20then separates the carbon dioxide from the water contained in theexhaust stream. The dry, substantially hydrogen free separated carbondioxide is then provided to the containment unit 21 for sequestration,and the separated water is available for humidification of the fuelinlet stream or other industrial uses. Thus, the environmentallyfriendly system preferably contains no burner and the fuel exhaust isnot combusted with air. The only exhaust from the system consists ofthree streams—water provided through conduit 23, sequestered carbondioxide provided through conduit 22 to tank 21, and oxygen and carbondioxide depleted air cathode exhaust stream through conduit 25. Thus,the system 100 generates electricity from a hydrocarbon fuel, whileoutputting substantially no pollutants into the atmosphere and cleaningthe air by removing carbon dioxide from the air exhaust stream. Thus,the system outputs cleaner air than it takes in, without releasingpollutants into the atmosphere.

The hydrogen separated from the fuel exhaust stream is then removed fromunit 1 through outlet 8 and conduit 7 and provided into the hydrocarbonfuel inlet stream in the fuel inlet conduit 111. If desired, prior tobeing provided to the fuel inlet conduit, the hydrogen stream may bepassed through a hydrogen cooler heat exchanger 129, where the hydrogenstream exchanges heat with an air stream, such as the air inlet streamprovided into the fuel cell stack 101. The temperature of the hydrogenstream is lowered in the heat exchanger 129 before being provided intothe fuel inlet conduit, while the temperature of the air inlet stream israised. Thus, the hydrocarbon fuel inlet stream is mixed with a low dewpoint, near ambient temperature recycled hydrogen recovered from theanode tail gas with an electrochemical hydrogen pump 1.

Thus, with respect to the fuel exhaust stream, the heat exchanger 121 islocated upstream of the reactor 128, which is located upstream of theheat exchanger 125, which is located upstream of the pump unit 1, whichis located upstream of the humidifier 119 and the fuel inlet conduit111.

If desired, all or a portion of the hydrogen separated from unit 1 maybe provided to a hydrogen using device, such as a PEM fuel cell in avehicle or another hydrogen using device or to a hydrogen storagevessel. In this case, a selector valve may be placed in conduit 7 toeither split the hydrogen stream between the fuel inlet conduit 111 andthe hydrogen storage vessel or hydrogen using device, or to alternatethe hydrogen flow between the fuel inlet conduit 111 and the hydrogenstorage vessel or hydrogen using device. Any component of the system 100may be operated by a computer or an operator to controllably vary thegas flow based on one or more of the following conditions: i) detectedor observed conditions of the system 100 (i.e., changes in the systemoperating conditions requiring a change in the amount of hydrogen in thefuel inlet stream); ii) previous calculations provided into the computeror conditions known to the operator which require a temporal adjustmentof the hydrogen in the fuel inlet stream; iii) desired future changes,presently occurring changes or recent past changes in the operatingparameters of the stack 101, such as changes in the electricity demandby the users of electricity generated by the stack, changes in price forelectricity or hydrocarbon fuel compared to the price of hydrogen, etc.,and/or iv) changes in the demand for hydrogen by the hydrogen user, suchas the hydrogen using device, changes in price of hydrogen orhydrocarbon fuel compared to the price of electricity, etc.

It is believed that by recycling at least a portion of the hydrogen fromthe fuel exhaust (i.e., tail) gas stream into the fuel inlet stream, ahigh efficiency operation of the fuel cell system is obtained.Furthermore, the overall fuel utilization is increased. For example, atleast 95% of the hydrogen in the fuel exhaust is separated by the unit 1and recycled back to the stack 101. Preferably at least 99%, such asabout 100%, of the fuel exhaust gas hydrogen is separated by the unit 1and recycled back to the stack 101. The “overall” or “effective” fuelutilization of a system having a given “per pass” fuel utilization rateis typically greater than the percentage of hydrogen recycled by such asystem. For example, a system that recycles only 85% of the fuel exhaustgas hydrogen has an effective fuel utilization of about 94% to about95%, if its per pass utilization is about 75%. Such a system would havean AC electrical efficiency of about 50% to about 60%, such as about 54%to 60%. In contrast, the instant system 100 includes a cascadedelectrochemical hydrogen pump unit 1, which recycles at least 95% ofexhaust hydrogen, and therefore operates at a higher overall fuelefficiency (greater than 95%) and a higher AC electrical efficiencygreater than 60%. Even higher efficiency may be obtained by increasingthe per pass fuel utilization rate above 75%, such as about 76-80%. Atsteady-state, there is no need for generating steam when steam methanereformation is used to create the feed gas to the fuel cell. The fuelexhaust stream contains enough water vapor to humidify the fuel inletstream to the stack at steam to carbon ratios of 2 to 2.5. The increasein overall fuel utilization and the removal of heat requirement togenerate steam increases the overall electrical efficiency. In contrast,a system that does not recycle any of the exhaust hydrogen has an ACelectrical efficiency of only about 45%, assuming equivalent per passutilization.

The fuel cell system described herein may have other embodiments andconfigurations, as desired. Other components may be added if desired, asdescribed, for example, in U.S. application Ser. No. 10/300,021, filedon Nov. 20, 2002 and published as U.S. Published Application No.2003/0157386, in U.S. Provisional Application Ser. No. 60/461,190, filedon Apr. 9, 2003, and in U.S. application Ser. No. 10/446,704, filed onMay 29, 2003 and published as U.S. Published Application No.2004/0202914, all of which are incorporated herein by reference in theirentirety. Furthermore, it should be understood that any system elementor method step described in any embodiment and/or illustrated in anyFIGURE herein may also be used in systems and/or methods of othersuitable embodiments described above, even if such use is not expresslydescribed.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedescription was chosen in order to explain the principles of theinvention and its practical application. It is intended that the scopeof the invention be defined by the claims appended hereto, and theirequivalents.

1. A method of operating a fuel cell system, comprising: providing ahydrocarbon fuel inlet stream and a carbon dioxide containing air inletstream into the fuel cell system, wherein the fuel cell system comprisesa fuel cell stack and a carbon dioxide pump; operating the system togenerate electricity and a plurality of output streams consistingessentially of oxygen and carbon dioxide depleted air, water, and carbondioxide available for sequestration, providing the carbon dioxidecontaining air inlet stream into the carbon dioxide pump; removing atleast 90 percent of the carbon dioxide contained in the air inlet streamusing the carbon dioxide pump; providing the air inlet stream from whichthe at least 90 percent carbon dioxide has been removed from the carbondioxide pump into the fuel cell stack; operating the fuel cell stack togenerate the electricity, a fuel exhaust stream and an oxygen and carbondioxide depleted air exhaust stream; providing at least a portion of thefuel exhaust stream into the carbon dioxide pump; and removing at leasta portion of the carbon dioxide contained in the air inlet stream usingthe fuel exhaust stream in the carbon dioxide pump.
 2. The method ofclaim 1, further comprising: providing the hydrocarbon fuel inlet streamtoward the fuel cell stack.
 3. The method of claim 2, wherein: the fuelcell stack comprises a SOFC stack; and the carbon dioxide pump comprisesa molten carbonate fuel cell stack.
 4. The method of claim 3, furthercomprising separating at least 95% of hydrogen contained in the fuelexhaust stream using a cascaded electrochemical hydrogen pump, andproviding the hydrogen separated from the fuel exhaust stream into thehydrocarbon fuel inlet stream.
 5. The method of claim 4, furthercomprising providing a remaining fuel exhaust stream consistingessentially of water and carbon dioxide from the cascadedelectrochemical hydrogen pump to a dryer, separating the water from thecarbon dioxide in the dryer and sequestering the separated carbondioxide.
 6. The method of claim 5, wherein all output streams of thesystem consist essentially of a first output stream consistingessentially of oxygen and carbon dioxide depleted air, a second outputstream consisting essentially of water, and a third output streamconsisting essentially of carbon dioxide provided for sequestration. 7.The method of claim 3, wherein the air inlet stream comprises at least0.02% carbon dioxide by volume.
 8. A fuel cell system, comprising: asolid oxide fuel cell stack; a carbon dioxide pump; a first conduitwhich operatively connects a fuel exhaust outlet of the fuel cell stackto a first inlet of the carbon dioxide pump; and a second conduit whichoperatively connects an air exhaust outlet of the carbon dioxide pump toan air inlet of the fuel cell stack.
 9. The system of claim 8, wherein:the carbon dioxide pump further comprises a second inlet exposed to anair atmosphere comprising carbon dioxide; and the carbon dioxide pump isadapted to remove at least a portion of the carbon dioxide in theatmosphere.
 10. The system of claim 9, wherein the carbon dioxide pumpis adapted to remove at least 90% of the carbon dioxide in theatmosphere comprising at least 0.02% carbon dioxide by volume.
 11. Thesystem of claim 8, further comprising a third conduit which operativelyconnects a fuel exhaust outlet of the carbon dioxide pump to an inlet ofa cascaded electrochemical hydrogen pump, a fourth conduit whichoperatively connects a first outlet of the cascaded electrochemicalhydrogen pump to a fuel inlet of the fuel cell stack and a fifth conduitwhich operatively connects a second outlet of the cascadedelectrochemical hydrogen pump to a dryer.
 12. The system of claim 11,further comprising a carbon dioxide storage tank operatively connectedto one output of the dryer.
 13. The system of claim 11, wherein thecascaded electrochemical hydrogen pump comprises a high temperature, lowhydration ion exchange membrane cell stack.
 14. The system of claim 8,wherein: the carbon dioxide pump comprises a molten carbonate fuel cellstack.
 15. A method of operating a fuel cell system, comprising:providing a hydrocarbon fuel inlet stream towards a solid oxide fuelcell stack; providing an air inlet stream and at least a portion of afuel exhaust stream of the solid oxide fuel cell stack to a moltencarbonate carbon dioxide pump to remove carbon dioxide from the airinlet stream; and providing a carbon dioxide depleted air stream fromthe carbon dioxide pump into the solid oxide fuel cell stack.